Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere.
A typical method of testing for the presence of leaks in an emission control system includes applying a vacuum to a fuel system that is otherwise sealed. An intact fuel system may be indicated if a threshold vacuum is met. In some examples, the fuel system may be sealed while containing a vacuum, and an intact fuel system may be indicated if the rate of vacuum bleed-up is less than a threshold. Failure to meet these criteria may indicate degradation in the fuel system. In some examples, an intake manifold vacuum may be used as the vacuum source applied to the emissions control system. However, hybrid-electric vehicles (HEVs) have limited engine run time, and may thus have limited opportunities to perform such a test. Further, in order to improve fuel efficiency, vehicles may be configured to operate with a low manifold vacuum, and may thus have limited opportunities with sufficient vacuum to perform a leak test.
In order to meet emissions regulations, such vehicles are required to include an on-board vacuum pump, which may be included in an evaporative leak check module (ELCM). The ELCM may be coupled to the evaporative emissions system, within a canister vent line, for example. The ELCM may thus supply the vacuum for appropriate leak tests. However, installing an ELCM in a vehicle is a relatively expensive manufacturing cost, which increases with a correlation to evaporative emissions system and fuel tank volume. Further, in applying a vacuum to the fuel tank, the ELCM draws fuel vapor into the fuel vapor canister. This may require an increase in canister size and/or the addition of an additional bleed canister in order to prevent bleed emissions in hybrid vehicles, which have limited opportunities to purge the canister for the same reasons an ELCM is required in the first place.
The inventors herein recognized the above stated problems and issues and have developed systems and methods in order to at least partially address them. In one example, a method for a fuel system is provided, comprising: during a first engine-off condition, coupling a fuel tank to a fuel vapor canister, and indicating degradation based on a change in pressure at the fuel vapor canister, and during a second engine-off condition, coupling the fuel vapor canister to an intake of an engine, and indicating degradation based on a change in pressure at the fuel vapor canister. The first engine-off condition may include an absolute fuel tank pressure greater than a threshold, while the second engine-off condition may include an engine spinning unfueled. In this way, a vacuum or pressure may be applied to the fuel vapor canister during an engine-off condition without requiring a dedicated vacuum pump coupled to the fuel vapor canister.
In another example, a method for a fuel system is provided, comprising: during a first condition, responsive to a first absolute fuel tank pressure being less than a threshold, maintaining a fuel tank sealed for a threshold duration; indicating degradation of the fuel tank responsive to a second absolute fuel tank pressure being less than the threshold; responsive to the first absolute fuel tank pressure being greater than the threshold, coupling a fuel tank to a fuel vapor canister; and indicating degradation based on a change in pressure at the fuel vapor canister. The method allows for passive testing of the fuel tank and fuel vapor canister using fuel tank pressure accumulated over a diurnal cycle. In this way, the canister side of an emissions control system may be tested without saturating the fuel vapor canister with hydrocarbons, thereby decreasing bleed emissions.
In yet another example, a system for a hybrid-electric vehicle is provided, comprising: a fuel tank coupled to a fuel vapor canister via a fuel tank isolation valve; an engine intake coupled to the fuel vapor canister via a canister purge valve; a canister vent coupling the fuel vapor canister to atmosphere via a canister vent valve; a fuel tank pressure sensor coupled to the fuel tank; a canister vent pressure sensor coupled within the canister vent; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: during a first engine-off condition, open the fuel tank isolation valve while maintaining the canister purge valve and canister vent valve closed, and indicate degradation based on a change in pressure at the canister vent pressure sensor; and during a second engine-off condition, open the canister purge valve while maintaining the canister vent valve and fuel tank isolation valve closed, and indicate degradation based on a change in pressure at the fuel vapor canister. By coupling separate pressure sensors to the fuel tank side and the canister side of the emissions control system, canister side leaks may be tested independently of the fuel tank pressure sensor. In this way, the fuel tank may remained sealed during some canister-side degradation tests, thereby maintaining fuel vapor isolated and reducing the transfer of fuel vapor to the fuel vapor canister.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.